I remember that the border around the screen was one colour, but if you used interrupts you could change the colour half way through the TV's scan so you could

I remember that the border around the screen was one colour, but if you used interrupts you could change the colour half way through the TV's scan so you could produce a "horizon" with one colour at the top and another below. I wrote something that would do this, but it used to crash randomly for no apparent reason.